1. Field of the Invention
The present invention relates to a projection exposure method and a system used therefor and more particularly, to a projection exposure method applicable to pattern transfer during a fabrication sequence of semiconductor integrated circuits or the like and a projection exposure system used for the method.
2. Description of the Prior Art
In some fabrication processes of semiconductor integrated circuits (ICs) where fine patterns are required to be formed, a projection exposure system has been widely used because of its high fabrication efficiency.
The resolution and the depth of focus (DOF) of a conventional projection exposure system change dependent on the numerical aperture (NA) and the exposure wavelength of a projection lens system used in the projection exposure system. If the NA is made large and/or the exposure wavelength is made short in order to improve the resolution, there arises a disadvantage that the DOF becomes shallow.
Conventionally, to make the DOF deeper without change or degradation in the resolution, a projection exposure method called as the "Focus Latitude enhancement Exposure (FLEX)" was developed, which is disclosed in the Japanese Non-Examined Patent Publication No. 63-42122. In the FLEX, multiple exposures are carried out in several different positions along the light axis of its optical projection system.
Additionally, to make the DOF deeper, another projection exposure method using a modified illumination was developed, which is disclosed in the Japanese Non-Examined Patent Publication No. 4-180612. In the method, there is provided a modification member for modifying an illumination beam of light in the optical path of the illumination beam.
With the conventional method named FLEX, a DOF can be increased for an isolated geometric shape of a mask pattern because of the large distances between the isolated geometric shape and the other adjacent geometric shapes thereof. However, for closely arranged geometric shapes of the mask pattern, the contrast of optical images of the closely arranged shapes becomes lower because of the small distances among the adjacent geometric shapes, so that the adjacent two ones of the geometric shapes cannot be resolved. Thus, it has been considered that the conventional method named FLEX is effective for only the isolated geometric shapes, so that the application field of the FLEX has been restricted to a projection exposure process for forming contact holes in several fabrication processes of semiconductor ICs.
With a conventional IC mask including geometric shapes for contact holes, the distances between adjacent contact holes or adjacent geometric shapes have been two times as large as the diameters or sizes of the contact holes, or more. Therefore, conventional IC masks contain patterns formed substantially of a plurality of isolated geometric shapes.
Recently, however, to increase the integration degree of semiconductor ICs, a mask whose pattern contains both isolated geometric shapes and closely arranged geometric shapes required to be used.
For example, as shown in FIG. 1, a semiconductor IC mask 34 has an isolated square shape 43 and a plurality of closely arranged square shapes 44. The shapes 43 and 44 are both used for contact holes. The distances or pitches between adjacent closely arranged shapes 44 are less than two times as large as the respective sizes of the shapes 44.
In the case of the mask 34, there arises a problem that the closely arranged square shapes 44 cannot be resolved when the conventional method named as FLEX is used to make the DOF of the isolated square shape 43 deeper.
On the other hand, with the conventional method using the modified illumination, the DOF for the closely arranged shapes 44 can be made deeper, however, the DOF for the isolated shape 43 is not made deeper. This means that the conventional method using the modified illumination cannot make the DOF large for all of the shapes 43 and 44.
FIG. 2 shows another IC mask 54 having a line-and-space pattern. The pattern is formed of an isolated linear shape 45 like a strip and a plurality of closely arranged linear shapes 46 like strips. The shapes 45 and 46 are used for electrical interconnection, respectively. The distances or pitches between the adjacent two closely arranged shapes 46 are less than two times as large as the respective widths of the linear shapes 46.
Also in the case of the mask 54, there arises the same problems as that of the mask 34 shown in FIG. 1.